1. Field of the Invention
This invention relates generally to surge detection devices and more particularly, it relates to a method and apparatus for detecting surge in a compressor of a compressor-driven system.
The invention is susceptible of widely diverse use and may be applied to any system having equipment driven by a compressor, such as in a chemical process, pump and the like.
2. Description of the Prior Art
As is generally known, surge or surging is an unstable condition that may occur when centrifugal compressors are operated at light loads and high pressure ratios. It is a transient phenomenon which is characterized by high frequency oscillations in pressures and flow and in some cases, there may even be a complete flow reversal through the compressor. Such continuous surging causes excessive vibrations in both the rotating and stationary components of the compressor, which may cause permanent damage thereto. Heretofore, surge detection devices of the prior art required measurements of the compressor head and suction flow, and surge was avoided by opening a bypass valve.
It has been discovered in the present invention that surge indirectly influences the drive system for the compressor. In particular, at surge conditions there exists a momentary reduction in flow and pressure developed across the compressor. Further, there will be a reduction in the net torque and mechanical power which are carried by the compressor drive shaft. In the case of where the drive device is an electric motor, these oscillations in torque and power will cause oscillations in motor current and electrical power consumption. It has been found that motor current and differential pressure change much more rapidly during surge than during any normal changes in capacity or load. As the load on the compressor increases, the rate of change of the differential pressure and of the motor current will also increase during a surge condition. The duration of the surge disturbance of motor current and differential pressure remain substantially constant over a wide range of loads and is substantially shorter than the duration of normal load fluctuations.
It will therefore be desirable to provide an improved surge detection arrangement for use in refrigeration systems having a centrifugal compressor driven by an electric motor which is based upon the rate of change of pressure developed across the compressor and the rate of change of the current drawn by the compressor drive motor.